The sounds of battle continued past daybreak, with residents staying indoors and only military and police vehicles seen patrolling the city centre. Roadblocks were set up in part of the city, residents said.

Military spokesman Colonel Gaspard Baratuza said the goal of the assailants was to steal weapons and use them to free prisoners.

Several hundred people have been imprisoned in Burundi for opposing President Pierre Nkurunziza's election this year to a third term , which many Burundians and foreign observers had opposed as unconstitutional and in violation of peace accords that ended a civil war in which 300,000 people were killed between 1993 and 2006.

The Constitutional Court ruled in favour of Mr Nkurunziza, who says he was entitled to another term because for his first term he was elected by parliament and not by popular mandate.

The deputy president of the Constitutional Court fled to exile in Rwanda before the ruling and said the court had been coerced to rule in favour of the president.

Mr Nkurunziza's push for a third term has triggered months of violence, including a coup attempt. At least 240 people have been killed since April and about 215,000 have fled to neighbouring countries, according to the UN.

At least seven people were killed earlier this week six by men wearing police uniforms.

Opponents and supporters of the government have been killed in apparent revenge attacks.

Burundi has a history of deadly conflicts between the country's Hutu and Tutsi ethnic groups.